1. Field of the Invention
This invention relates to an engine exhaust emission control system designed to reduce noxious substances from exhaust emissions.
2. Description of the Related Art
Catalytic converter systems are generally used to control exhaust emissions from an automotive engine operating with an air fuel mixture having the stoichiometric ratio. During the engine start-up period, fuel is not, however, effectively mixed with air, and too much fuel tends to be supplied so that the air-fuel mixture is rich. Therefore, raw hydrocarbons are introduced into the atmosphere before the catalytic converter systems reach their effective temperature during engine start-up.
There have been numerous suggestions for avoiding the foregoing problem. For example, it has been proposed to dispose in the exhaust system adsorbents such as active carbon and zeolite so as to adsorb hydrocarbons. European patent application EP 424,966 A exemplifies an exhaust gas control device, which comprises a catalytic converter, an adsorbent sieve adsorptive of hydrocarbons and located upstream of a catalytic converter, a bypass passing through the adsorbent sieve, and a change-over valve, thereby adsorbing, desorbing and oxidizing hydrocarbons. U.S. Pat. No. 5,125,231 suggests an engine exhaust system to reduce hydrocarbon emissions, which comprises a first catalytic converter, an adsorbent sieve adsorptive of hydrocarbon and a second catalytic converter both of which are located downstream of the first catalytic converter, a bypass line detouring the adsorbent sieve and the second catalytic converter, and a recycle line for connecting an exhaust line upstream of the first converter and the exhaust line downstream of the second converter, thereby adsorbing, desorbing and oxidizing hydrocarbons. Further, SAE TECHNICAL PAPER SERIES 920847 describes an exhaust emission control system, which comprises a catalytic converter, an adsorbent sieve located downstream of the catalytic converter, an adsorbent bypass, an exhaust line between an intake pipe and an exhaust pipe, three valves and an air pump so that hydrocarbons are adsorbed, desorbed and oxidized.
Unfortunately, such conventional devices suffer from problems that adsorbed hydrocarbons sometimes desorb from the adsorbent sieve before the catalytic converters reach their effective temperature, or that a very complicated control mechanism is required for operating a plurality of valves and the air pump.